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@PebbleChamp

That's only true for Thunderbird versions before 3. There was a bug that previously prevented the messages from being sorted properly even after changing the setting you point out:

http://micropipes.com/blog/2008/10/22/threadbubble-going-the-way-of-the-dodo/

It's a shame to give someone's work 1 star without checking into the situation, it was a bug that lasted nearly 4 years and his addon was very useful for people for a long time because of this.